电气外文翻译电力系统故障Word下载.docx

电气外文翻译电力系统故障Word下载.docx

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Faultsonpowersystem

Eachyearnewdesignsofequipmentbringaboutincreasedreliabilityofoperation.Nevertheless,equipmentfailuresandinterferencebyoutsidesourcesoccasionallyresultinfaultsonelectricpowersystem.Ontheoccurrenceofpowerfromthegeneratingstationstotheloadsmaybeunsatisfactoryoveraconsiderablearea,andifthefaultedequipmentisnotpromptlydisconnectedfromtheremainderofthesystem,damagemayresulttootherpiecesofoperatingequipment.

Afaultistheunintentionalorintentionalconnectingtogetheroftwoormoreconductorswhichordinarilyoperatewithadifferenceofpotentialbetweenthem.Theconnectionbetweentheconductionsmaybebyphysicalmetalliccontactoritmaybethroughanarc.Atthefault,thevoltagebetweenthetwopartsisreducedtozerointhecaseofmetal-to-metalcontacts,ortoaverylowvalueincasetheconnectionisthroughanarc.Currentsofabnormallyhighmagnitudeflowthenetworktothepointoffault.Theseshort-circuitcurrentswillusuallybemuchgreaterthanthedesignedthermalabilityoftheconductorsinthelinesormachinesfeedingthefault.Theresultantriseintemperaturemaycausedamagebytheannealingofconductorsandbythecharringofinsulation.Intheperiodduringwhichthefaultispermittedtoexist,thevoltageonthesysteminthenearvicinityofthefaultwillbesolowthatutilizationequipmentwillbeinoperative.Itisapparentthatthepowersystemdesignermustanticipatepointsatwhichfaultmayoccur,beabletocalculateconditionsthatexistduringafault,andprovideequipmentproperlyadjustedtoopentheswitchesnecessarytodisconnectfaultedequipmentfromtheremainderofthesystem.Ordinarilyitisdesirablethatnootherswitchesonthesystemareopened,assuchbehaviorwouldresultinunnecessarymodificationofthesystemcircuits.

Adistinctionmustbemadebetweenafaultandanoverload.Anoverloadimpliesonlythatloadsgreaterthanthedesignedvaluehavebeenimposedonsystem.Undersuchacircumstancethevoltageattheoverloadpointmaybelow,butnotzero.Thisundervoltageconditionmayextendforsomedistancebeyondtheoverloadpointintotheremainderofthesystem.Thecurrentsintheoverloadedequipmentarehighandmayexceedthethermaldesignlimits.Nevertheless,suchcurrentsaresubstantiallylowerthaninthecaseofafault.Servicefrequentlymaybemaintained,butatbelow-standardvoltage.

Overloadsarerathercommonoccurrenceinhomes.Forexample,ahousewifemightplugfivewaffleironsintothekitchencircuitduringaneighborhoodparty.Suchanover-load,ifpermittedtocontinue,wouldcauseheatingofthewiresfromthepowercenterandmighteventuallystartafire.Topreventsuchtrouble,residentialcircuitsareprotectedbyfuseorcircuitbreakerswhichopenquicklywhencurrentsabovespecifiedvaluespersist.Distributiontransformersaresometimesoverloadedascustomersinstallmoreandmoreappliances.Thecontinuousmonitoringofdistributioncircuitsisnecessarytobecertainthattransformersizesareincreasedasloadgrows.

Faultsofmanytypesandcausesmayappearonelectricpowersystems.manyofusinourhomeshaveseenfrayedlampcordswhichpermittedthetowconductorsofthecordtocomeincontactwitheachother.Whenthisoccurs,thereisaresultingflash,andifbreakerorfuseequipmentfunctionsproperly,thecircuitisopened.

Overheadlines,forthemostpart,areconstructedofbareconductors.Thesearesometimesaccidentallybroughttogetherbyactionofwing,sleet,trees,cranes,airplanes,ordamagetosupportingstructures.Overvoltagesduetolightningorswitchingmaycauseflashoverofsupportingorfromconductortoconductor.Contaminationoninsulatorssometimesresultsinflashoverevenduringnormalvoltageconditions.

Theconductorsofundergroundcablesareseparatedfromeachotherandfromgroundbysolidinsulation,whichmaybeoil-impregnatedpaperoraplasticsuchaspolyethylene.Thesematerialsundergosomedeteriorationwithage,particularlyifoverloadsonthecableshaveresultedintheiroperationatelevatedtemperature.Anysmallvoidpresentinthebodyoftheinsulatingmaterialwillresultinionizationofthegascontainedtherein,theproductsofwhichreactunfavorablywiththeinsulation.Deteriorationoftheinsulationmayresultinfailureofthematerialtoretainitsinsulatingproperties,andshortcircuitswilldevelopbetweenthecableconductors.Thepossibilityofcablefailureisincreasediflightningorswitchingproducestransientvoltageofabnormallyhighvaluesbetweentheconductors.

Transformerfailuresmaybetheresultofinsulationdeteriorationcombinedwithovervoltagesduetolightningorswitchingtransients.Shortcircuitsduetoinsulationfailurebetweenadjacentturnsofthesamewindingmayresultfromsuddenlyappliedovervoltages.Majorinsulationmayfail,permittingarcstobeestablishedbetweenprimaryandsecondarywindingsorbetweenawindingandgroundedmetalpartssuchasthecoreortank.

Generatorsmayfailduetobreakdownoftheinsulationbetweenadjacentturnsinthesameslot,resultinginashortcircuitinasingleturnofthegenerator.Insulationbreakdownmayalsooccurbetweenoneofthewindingsandthegroundedsteelstructureinwhichthecoilsareembedded.Breakdownbetweendifferentwindingslyinginthesameslotresultsinshort-circuitingextensivesectionsofmachine.

Balancedthree-phasefaults,likebalancedthree-phaseloads,maybehandledonalineto-neutralbasisoronanequivalentsingle-phasebasis.Problemsmaybesolvedeitherintermsofvolts,amperes,andohms.Thehandlingoffaultsonsingle-phaselinesisofcourseidenticaltothemethodofhandlingthree-phasefaultsonanequivalentsingle-phasebasis.

Voltagetransformers

Voltagetransformersareusedwithvoltmeters,watt-meters,watt-hourmeters,power-factormeters,frequencymeters,synchroscopesandsynchronizingapparatus,protectiveandregulatingrelays,andtheno-voltageandover-voltagetripcoilsofautomaticcircuitbreakers.Onetransformercanbeusedforanumberofinstrumentsatthesametimeifthetotalcurrenttakenbytheinstrumentsdoesnotexceedthatforwhichthetransformerisdesignedandcompensated.

Voltagetransformersaregenerallydesignedforacapacityofabout200volt-amp.Therearetwocausesoferrorsinvoltagetransformers,namely,ratioerrorandphase-angleerror.Thepartoftheseerrorsduetotheexcitingcurrentisconstantforanyparticularvoltage.Itcanbereducedtoaminimumbychoosingthebestqualityofironandworkingitatalowmagneticdensity.Thepartoftheerrorsduetotheloadcurrentvariesdirectlywiththeloadandcanbeminimizedbymakingtheresistanceofthewindingsveryslow.

Voltagetransformersarecompensatedfortheirironlossesatratedvoltage.Whenusedonsomeothervoltage,eitherhigherorlower,anerrorisintroduced.Ingeneralthiserrorwillnotbemorethan0.15percentofratedvoltage.Avoltagetransformershouldneverbeusedonacircuitwhosevoltageismorethan10percentabovetheratedvoltageofthetransformer.

Thesecondaryterminalsofavoltagetransformershouldneverbeshort-circuited,aheavycurrentwillflowwhich,ifcontinued,willburnoutthewindings.Inordertoprotectthesystemagainstsustainedshortcircuitsinthetransformercircuit,itisgenerallyrecognizedasgoodpracticetointroduceintotheprimarycircuitaresisterandfuse,thesebeenconnectedinseries.Theresistorsaredesignedtolimitthecurrenttoabout20to40amp.,whilethefusesaredesignedtobreaksuchcurrent.Innormaloperationthecurrentwhichtheresistorcarriesisonlytheverysmallprimarycurrentofthevoltagetransformer,andthedropinvoltagethattheycauseisinappreciable.

Currenttransformers

Currenttransformersareusedwithammeters,watt-meters,power-factormeters,watt-hourmeters,compensators,protectiveandregulatingrelays,andthetripcoilofcircuitbreakers.Onecurrenttransformercanbeusedtooperatenottoexceedthatforwhichthetransformerisdesignedandcompensated.

Thecurrenttransformerisconnecteddirectlyinserieswiththeline,andusuallyhasafixednumberofinstrumentsinthesecondary.Ariseorfallinthelinecurrentrequiresacorrespondingriseorfallinthesecondaryvoltagetoforcethesecondarycurrentthroughtheimpedanceofthemeterload.themagneticfluxintheiron,whichsuppliesthevoltage,thusfollowstheriseandfalloftheprimaryorlinecurrent.

Theinstrumentsconnectedinthesecondarycircuitofthetransformerareplacedinseries,sothatthesecondarycurrentwillpassthrougheachinstrument.Astheinstrumentareadded,highervoltageisrequiredtoforcethecurrentthroughtheinstruments.Thisrequiresahighmagneticdensityintheiron.Ahighermagneticdensityincreasesboththeironlossandthemagnetizingcurrent;

henceboththeratioandthephase-angleerrorsaremagnified.Forthesakeofaccuracy,therefore,thereisalimittothenumberofinstrumentsthatshouldbeplacedonasinglecurrenttransformer.

Thesecondarycircuitofacurrenttransformershouldneverbeopenedwhiletheprimaryiscarryingcurrent.Ifitisnecessarytodisconnectinstruments,thesecondaryshouldfirstbeshort-circuited.Ifthesecondarycircuitisopened,adifferenceofpotentialisdevelopedbetweenterminalswhichisdangeroustoanyonecomingincontactwiththemetersofleads.Thecauseofthishighvoltageisthatwithopensecondarycircuitalltheprimaryampereturnsareeffectiveinproducingfluxinthecore,whereasnormallybutasmallportionofthe